An example of a machine tool control system is shown in U.K. patent specification No. 2143968 corresponding to U.S. Pat. No. 4,672,550. In this system the tool is controlled by a digital computer which produces digital signals corresponding to the tool positions necessary to form the required profile on the workpiece. The system disclosed, however, is only partly digital. The digital signals are converted into analogue form for controlling the tool position and the system incorporates an analogue feedback loop responsive to analogue signals dependent on the position of the tool. Such a system has the disadvantages inherent in its use of analogue signals. It is an object of the present invention to provide a machine tool control system having the full advantages of digital systems (simpler processing, greater accuracy and freedom from drift).